Bug 12543 [1] requests that step 12 (replacing "\" with "/") of the "resolve a URL" algorithm be removed, as Gecko behavior has shown that it isn't needed for compatibility. The discussion in a related bug 14693 [2] also highlights other problems with step 12 and the algorithm in general. It has been suggested that the entire section of URL parsing be removed and instead defer to the URL spec [3].
Are there any objections to this course of action? If not, I'll take a stab at removing the relevant sections and stage them in a branch before committing to HTML5.1 and resolving 12543 as fixed.
Alternatively, we could try to fix the URL parsing algorithm within the HTML5.1 spec.
Thanks,
Erika Navara
[1] https://www.w3.org/Bugs/Public/show_bug.cgi?id=12543
[2] https://www.w3.org/Bugs/Public/show_bug.cgi?id=14693
[3] https://dvcs.w3.org/hg/url/raw-file/tip/Overview.html